This invention is directed toward a method for recharging electrochemical battery systems. More precisely, a method suitable for use in systems characterized by high depth discharge maintenance free batteries.
Electric utility power grid applications utilize batteries to store energy during off-peak demand periods of operation for subsequent discharge during peak demand periods. Similarly, high power usage customers may use similar systems to advantageously store energy during off-peak periods for use later in order to take advantage of lower rates during the off-peak periods. Such systems are typified by deeply discharged, relatively large flooded cell batteries. Each battery may have plates upwards of three feet in height with a correspondingly tall battery case and may occupy a large footprint. These batteries are often times custom designed and hand built which results in substantial cost penalties associated therewith.
It is well recognized that, especially with deep discharges, the electrolyte in flooded cell batteries undergoes considerable stratification upon recharge. Left unaddressed, electrolyte stratification will result in plate damage due to the higher acid concentration of electrolyte reacting corrosively at the bottoms thereof. Additionally, there will exist a charge gradient from top to bottom of the plates which may cause early termination of a recharge. The battery thereby never reaches its full capacity and suffers from gradual capacity losses throughout its life. These problems may be more pronounced with larger batteries as described.
One manner of dealing with electrolyte stratification which has been practiced is to introduce mechanical agitation to the electrolyte. Bubblers have been installed in battery cells but adds greatly to cost, complexity and maintenance required. Another alternative is to recharge the battery so that gassing occurs and destratifies the electrolyte. Of course, this is not wholly desirable because it requires certain additional burdens. Water losses associated with gassing results in a battery system that requires an expensive and often unreliable watering system or a maintenance free battery system with a greatly reduced cycle life due to irreplaceable water losses.
A further drawback to flooded cells is that the gas recombination is not as efficient as in starved electrolyte batteries due to lack of an efficient transport medium such as a glass mat between the plates. Therefore, cycle life of these flooded cells are very sensitive to excess gas evolution since more of the gas evolved will be dissipated out of the system resulting in water losses which are not replaceable. For these reasons, conventional recharge methods rely to a great extent upon absolute minimal gas evolution through precise voltage control especially toward the end of recharge where the effects of too high a voltage upon gas evolution are most pronounced. Of course, too conservative a voltage controlled recharge undesirably results in excessive charge time.
Therefore, in terms of a wholly maintenance free battery system, the cycle life limiting factors and eventual causes of battery failure are water loss through gassing or stratification and associated plate damage.